The present invention relates to radio telephone equipment and, more particularly, to radio telephone equipment of the type communicating with other parties under the control of a base station.
In a vehicle telephone system, portable telephone system or similar radio telephone system, radio telephone equipment such as a vehicle telephone terminal or a portable telephone set interchanges various kinds of control signals with a base station over a radio channel. The radio telephone system, therefore, needs an interval as long as several seconds to several tens of seconds between the time when the user manipulates keys of the telephone equipment for originating a call and the time when the user is actually allowed to begin a conversation with the other subscriber or party.
During this time interval, the user who operated the equipment for call origination can see that the call has been connected to the other party, by monitoring sound emitted by a loudspeaker which is installed in the equipment in addition to a microphone and a receiver for conversation. Then, the user picks up the handset or the portable telephone set to begin a conversation with the other party. Such a monitoring function is generally referred to as a loudspeaker receiving function.
The loudspeaker receiving function stated above makes it unnecessary for the user who originated a call to continuously hold the handset or the portable telephone until the user recognizes the voice of the other party which will be produced from the loudspeaker. However, if the volume of the sound monitored through the loudspeaker is small or the quality of the sound is poor, it is likely that engine noise and ambient noise coming into the vehicle cabin will prevent the user from timely recognizing that the other party has responded to the call. It has been customary, therefore, to use a loudspeaker which is about 50 millimeters in diameter and about 20 millimeters in thickness. A loudspeaker having such large dimensions is not feasible for portable telephone equipment whose space is at a decided premium.